.maintodo {
  width: 500px;
  margin: 100px auto;
}
header {
  background-color: black;
  color: white;
  height: 90px;
  /*弹性盒模型*/
  display: flex;
  /*主轴居中对齐*/
  justify-content: center;
  /*侧轴居中对齐*/
  align-items: center;
}

header h1 {
  float: left;
  margin: 20px;
}
header input {
  float: right;
  /* width: 100%; */
  height: 32px;
  margin: auto 20px;
}

h2 {
}
